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VM snapshots are snapshots of the complete virtual machine including
CPU state, RAM, device state and the content of all the writable
disks. In order to use VM snapshots, you must have at least one non
removable and writable block device using the qcow2
disk image
format. Normally this device is the first virtual hard drive.
Use the monitor command savevm
to create a new VM snapshot or
replace an existing one. A human readable name can be assigned to each
snapshot in addition to its numerical ID.
Use loadvm
to restore a VM snapshot and delvm
to remove
a VM snapshot. info snapshots
lists the available snapshots
with their associated information:
(qemu) info snapshots Snapshot devices: hda Snapshot list (from hda): ID TAG VM SIZE DATE VM CLOCK 1 start 41M 2006-08-06 12:38:02 00:00:14.954 2 40M 2006-08-06 12:43:29 00:00:18.633 3 msys 40M 2006-08-06 12:44:04 00:00:23.514
A VM snapshot is made of a VM state info (its size is shown in
info snapshots
) and a snapshot of every writable disk image.
The VM state info is stored in the first qcow2
non removable
and writable block device. The disk image snapshots are stored in
every disk image. The size of a snapshot in a disk image is difficult
to evaluate and is not shown by info snapshots
because the
associated disk sectors are shared among all the snapshots to save
disk space (otherwise each snapshot would need a full copy of all the
disk images).
When using the (unrelated) -snapshot
option
(disk_images_snapshot_mode), you can always make VM snapshots,
but they are deleted as soon as you exit QEMU.
VM snapshots currently have the following known limitations: